CsI doped with thallium is one of the most important scintillators currently in use. In terms of conversion of ionizing radiation to visible photons, it is the unquestioned leader in efficiency. Additionally, its high density, high atomic number, excellent light transport properties, high radiation resistance, easy availability, and low cost make it suitable for a wide variety of applications such as medical imaging and high energy physics. The one property of CsI(Tl) that limits its use in fast or time resolved applications is its afterglow. This causes problems in high count-rate applications, reconstruction artifacts in CT usage, and problems of reduced contrast and image blurring in high-speed imaging. If the afterglow in CsI(Tl) is quenched, this material could be effectively used in all these applications resulting in improved system performance with dramatic reduction in costs. To address this issue we propose to minimize the CsI(Tl) afterglow to a level where it can be suitably used for many technologically and commercially important applications, without sacrificing its excellent properties. Our approach during Phase II will involve detailed investigation of the scintillation processes in the CsI(T1) and quenching the afterglow by the addition of activators into its lattice. Potential Applications: The CsI(Tl) scintillator is widely used in many commercial products such as digital mammography systems, image intensifier tubes, and numerous other systems for medical and non-medical use. Reduction of afterglow will allow the use of this scintillator in many important imaging modalities such as medical CT, stereo-tactic mammography, and other high-speed imaging as well as counting applications. The use of CsI(Tl) in these systems is expected to substantially enhance their performance with dramatic reduction in cost.